Well,
yesterday I tested ESS stage 2 but can honestly say I did not notice any major difference in power. I did notice a very unpleasant jerkyness between gears 1 and 2, maybe due to the increased torque.

Not normal! I got back on stage 2 as my brother was visiting from Scandinavia two weeks ago, and its smooth as silk with my DCT. I get a lot of wheel spin, but no unpleasant jerks between any gears. I would have this checked before installing more mods!

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Little known facts about BMW: 1) The N54 is still in production and used in the current Z4 35i and 35is. 2) In a Z4 you can get sport+ steering combined with normal suspension etc by selecting DSC=off. (Update: DSC=off also offers sports steering in the M135i) 3) All Z4s with 19" wheels are still only offered with the worst performing OEM tires ever sold with a DCT equipped BMW (Potenza RE 050 RFT).